1. Field of the Invention
The present invention relates to a transmitter apparatus used in a non-priority communication system that uses a second frequency band adjacent to a first frequency band utilized in a priority communication system.
Further, the present invention relates to a mobile communication system, a base station, and a communication enable signal transmitter unit, which include a first priority communication system performing a first priority communication by using a first frequency band, a second priority communication system performing a second priority communication by using a second frequency band and has a priority level lower than the first priority communication, and a non-priority communication system by using a third frequency band, in which the first frequency band, the second frequency band, and the third frequency band are adjacent to each other.
In addition, the present invention relates to a communication enable signal transmitter apparatus and a priority communication system performing a priority communication by using the first frequency band and a non-priority communication system by using the second frequency band, in which the first frequency band and the second frequency band are adjacent to each other.
2. Description of the Related Art
Conventionally, there is known a communication system in which a mobile station is configured to transmit signals by using a frequency channel of a specific frequency band, when a transmission or a reception of the signals to and from other mobile stations are not detected in the specific frequency channel. (see Non-Patent literature 1: IEEE802.11h, “Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications” 2003).
In recent years, in order to utilize frequency bands effectively, there has been designed a network in which a priority communication system that performs a priority communication by using a frequency channel of a first frequency band, and a non-priority communication system that performs a communication by using a second frequency band adjacent to the first frequency band, coexist in the same communication area.
This network is supposed to prioritize communications of a mobile station for priority communications (hereinafter referred to as a priority communication mobile station) over communications of a mobile station for non-priority communications (hereinafter referred to as a non-priority communication mobile station).
However, in the above-described network, there has been a problem that the priority communication mobile station that desires to perform a communication by using the frequency channel of the first frequency band, when the non-priority communication mobile station transmits a signal by using the frequency channel of the first frequency band and when a transmission or a reception of the signals to and from the priority communication mobile station are not detected.
Moreover, as shown in FIG. 1, when a signal from a priority communication base station 10A does not reach a non-priority communication mobile station 30B due to a long distance between the priority communication base station 10A and the non-priority communication mobile station 30B, the non-priority communication mobile station 30B fails to detect communications performed in the priority communication system. Accordingly, this network has another problem that signals transmitted from the non-priority communication mobile station 30B may act as interference signals to a priority communication mobile station 30A.
In addition, as shown in FIG. 2, when a transmission power of signals transmitted from the priority communication mobile station 30A is weaker than a transmission power of signals transmitted from the non-priority communication mobile station 30B, the non-priority communication mobile station 30B fails to detect the signals transmitted from the priority communication mobile station 30A. In such a case, a problem also arises that signals transmitted from the non-priority communication mobile station 30B may act as interference signals to the priority communication mobile station 30A.
Further, when the network includes a plurality of the non-priority communication mobile stations 30B, the non-priority communication mobile stations 30B is required to extract and detect the signals transmitted from the priority communication mobile station 30A among interference signals transmitted from each of the plurality of the non-priority communication mobile stations 30B. Here, a problem also arises that the non-priority communication mobile station 30B may fail to start a communication by using the frequency channel of the second frequency band when a detection accuracy of the non-priority communication mobile station 30B deteriorates.
Furthermore, when the priority communication base station 10A constantly transmits a control signal in the network, the non-priority communication mobile station 30B may also fail to start the communication to by using the frequency channel of the second frequency band.
The present invention has been made in consideration of the foregoing problems, and has an object to provide a transmitter apparatus, a mobile communication system, a base station and a communication enable signal transmitter unit, which can prioritize communications of a mobile station in a priority communication system over communications of a mobile station in a non-priority communication system, in a network where the priority communication system and the non-priority communication system coexist in the same communication area. Here, the priority communication system performs a communication by using a frequency channel of a first frequency band while the non-priority communication system performs a communication by using of a frequency channel of a second frequency band adjacent to the first frequency band.